Acute leukemias that bear chromosomal translocations at 11q23 possess rearrangements of the Mixed Lineage Leukemia gene (MLL, HRX, ALL-1). More than 40 different MLL translocations have been reported, but the t(9;11) (MLL-AF9) and the t(4;11) (MLL-AF4) are particularly common. MLL-AF9 is most frequently identified in leukemias diagnosed as AML whereas MLL-AF4 is found solely in leukemias diagnosed as ALL or mixed-lineage leukemia. Patients with MLL-AF4 rearranged leukemias have a poor prognosis. This is particularly true for infant leukemia where approximately 80% of cases will harbor rearrangement of the MLL gene. We have previously demonstrated that human lymphoblastic leukemias harboring MLLrearrangements possess a unique gene expression profile that suggests they arise from an early hematopoietic progenitor. This approach also identified the receptor tyrosine kinase FLT3 as a potential therapeutic target in this disease. Using a murine model system of MLL-AF9 induced AML, we have recently identified a hematopoietic stem cell (HSC) associated gene expression program activated in granulocyte macrophage progenitors (GMP) during their conversion to leukemia stem cells (LSC). In these studies we noted repression of cebp/a as a component of the MLL-AF9 induced program. In specific aim 1 we will asses specific genes found highly expressed in LSC, and perform a high-throughput screen to identify small molecules that reverse the LSC program. In specific aim 2 of this proposal we will work with members of project 3 to interrogate the role of cebp/a in MLL-rearranged leukemia. In specific aim 3 we will work with members of projects 1 and 2 to develop a conditional murine model of MLL-AF4 induced leukemia and assess the potential for cooperation with mutant FLT3 alleles during leukemogenesis. Such a model will not only allow identification of progenitor populations susceptible to MLL-AF4 induced transformation, but also provide a much-needed model for assessment of therapeutics developed in projects 1 and 2.